Fluororubbers are known to have the following properties:
(1) having rubber elasticity which is a property intrinsic to rubber, like other general-purpose rubbers;
(2) having high chemical stability because the carbon chains therein are linked such that hydrogen atoms bonded to the carbon atoms are substituted by fluorine and also have no unsaturated bond; and
(3) being superior to other general-purpose rubbers in properties such as heat resistance, oil resistance, chemical resistance, weather resistance, and ozone resistance.
With such properties, fluororubbers are used, for example, for rubber seals such as O-rings, formed mainly of synthetic rubber, containing rubber chemicals such as a crosslinking agent, a crosslinking aid, and a filler, and molded using a mold by hot pressing.
Such fluororubber seals are widely used for apparatuses and equipment in various industries because those seals are so flexible as to conform well to joined surfaces (such as flange surfaces) and have high sealability.
In particular, fluororubber seals are used in the semiconductor industry as, for example, seals in chemical lines for processes such as etching processes because fluororubber seals have high heat resistance and chemical resistance and produce only small amounts of particulates, or called particles, and gas.
Processes involving plasma treatment in semiconductor manufacturing use plasma gases such as O2, CF4, O2+CF4, N2, Ar, H2, NF3, CH3F, CH2F2, C2F6, Cl2, BCl3, tetraethoxysilane (TEOS), and SF6. In particular, etching processes mainly use fluorocarbon gases, and ashing processes use mixed gases based on oxygen gas.
Seals conventionally used under plasma gas conditions are mainly formed from perfluoroelastomers (crosslinkable fluoropolymers fluorinated such that hydrogen atoms (H) in the side chains bonded to the main-chain C—C bonds are substantially completely substituted by fluorine atoms (F): classified as FFKM) for their high plasma resistance.
Perfluoroelastomers, however, are extremely expensive. Accordingly, less expensive crosslinkable fluoropolymers (unvulcanized fluororubber polymers in which hydrogen atoms (H) in the side chains bonded to the main-chain C—C bonds are incompletely substituted by fluorine atoms (F) and which partially contain hydrogen atoms (H): classified as FKM) have increasingly been used as seals for semiconductor manufacturing apparatuses at sites to be sealed in mild plasma environments.
However, seals have recently been used in severer environments with shrinking semiconductor design rules and increasing throughput (amount processed per unit time).
A seal is often attached to the place to be sealed so as to be stretched to some extent, for example, for ease of attachment and prevention of detachment, and also to be compressed for sufficient sealability.
If the seal is exposed to a plasma irradiation atmosphere, the rubber is degraded by plasma irradiation, and its weight decreases as the rubber component volatilizes from the surface of the seal.
In addition, the seal tends to crack when irradiated with plasma, and if the crack is severe, it may cause the seal to fracture. This causes problems such as increased maintenance operations for semiconductor manufacturing apparatuses due to shortened seal lives with degraded seal performance.
In general, a seal is often attached to a groove in a slightly stretched state; this stretching further promotes cracking. In addition, if a crack reaches the seal surface beyond the site being irradiated with plasma, it causes leakage and brings serious problems, such as a system failure, in manufacturing processes, thus leading to extensive damage.
Furthermore, fluororubbers conventionally used for seals contain a filler such as carbon black, silica, barium sulfate, or titanium oxide for improved properties at normal state, including mechanical strength and compression set, or as an extender for cost reduction.
In polyol vulcanization and amine vulcanization, additionally, a vulcanizing agent needs to be used in combination with a compound of metal such as a magnesium, lead, calcium, aluminum, or zinc as an acid acceptor. In polyol vulcanization, furthermore, a calcium hydroxide needs to be used in combination as a cocatalyst.
Such inorganic fillers contribute to production of particles (particulates) in plasma irradiation equipment, whereas it is difficult to form a seal having necessary properties at normal state using a fluororubber formed without an inorganic filler for reduction of particles produced.
In addition, because a deteriorated seal itself can produce particles even if no inorganic filler is used, a reduction in the amount of particles produced, in other words, an improvement in plasma resistance, is demanded of organic polymer materials used for seals themselves.
Accordingly, there has been a desire for a fluororubber seal having not only high plasma resistance (associated with the rate of weight loss), but also high crack resistance upon plasma irradiation and compression set (both associated with seal life), producing no particles, and being less expensive and more durable.
JP-A No. 6-302527 (Patent Document 1) proposes a silica-containing seal for semiconductor manufacturing apparatuses. The seal for semiconductor manufacturing apparatuses, however, has a problem in that it has insufficient crack resistance and produces particles originating from silica.
In addition, JP-A No. 2001-114964 (Patent Document 2) proposes a rubber composition, for a plasma-resistant seal, containing a fluororubber and a polyamine crosslinking agent, and JP-A No. 2001-164066 (Patent Document 3) proposes a rubber composition in which a polyamine crosslinking agent, and a polyol crosslinking agent are added to a fluororubber. These rubber compositions, however, have a problem in that they have insufficient plasma resistance and the metal oxide used as an acid acceptor for vulcanization contributes to production of particles.
Furthermore, JP-A No. 2007-137994 (Patent Document 4) proposes a fluorine-containing alloyed copolymer containing a fluororubber and resinous tetrafluoroethylene-perfluoroalkyl vinyl ether. The fluorine-containing alloyed copolymer, however, has a problem in that it has insufficient crack resistance and therefore fails to provide a sufficient seal life.
In addition, JP-A No. 2006-228805 (Patent Document 5) proposes a seal, for semiconductor manufacturing apparatuses, formed from a perfluoroelastomer blended with FKM for improved crack resistance. The seal for semiconductor manufacturing apparatuses, however, has a problem in that it has insufficient crack resistance and is more expensive than those formed only of a fluororubber because it uses a perfluoroelastomer.
An object of the present invention is to solve the above problems accompanied with the conventional art.